Olefinic products such as propylene, butylene, isobutylene, butadiene, pentene, pentadiene, and isopentadiene (isoprene) can be commercially produced by the vapor-phase catalytic dehydrogenation of propane, butanes, pentanes, or mixtures thereof. Two commercial processes for the production of olefins, for example, are the CATOFIN.RTM. and CATADIENE.RTM. processes marketed by Air Products and Chemicals, Inc., the assignee of the present invention. Typical dehydrogenation reactor vapor product contains olefinic products, unreacted and other paraffinic feed components, and various lower-boiling components including hydrogen. In the present invention, desired olefinic products are recovered from such dehydrogenation reactor products at a maximum of recovery and a minimum of capital and operating cost.
A conventional method for recovering olefinic products from dehydrogenation reactor product includes an absorption step wherein desired olefinic products as well as lower-boiling components are absorbed in a heavier hydrocarbon oil and subsequently are recovered and separated by stripping and distillation. While conventional gas plants including such an absorber-stripper separation step give a relatively clean split between the desired olefinic products and lower-boiling components, this separation method is energy intensive and thus has been largely supplanted by more economical low-temperature gas plants wherein product recovery is effected by means of compression, cooling, condensation, phase separation, and direct distillation, without the use of absorber-stripper units. In such low-temperature gas plants the feed gas is compressed, and the compressed vapor stream containing desired olefinic products and lower-boiling components is cooled by refrigeration to condense most of the desired products plus some lower boiling components. This condensate is then combined with olefin-rich condensate from the compression step and is separated by distillation to yield a liquid olefinic product and a vapor overhead containing mostly lower-boiling components and a small but significant amount of the desired olefinic products. To increase recovery of these valuable olefinic products, for example in the production of butadiene, this vapor overhead is recycled to the compression step, recompressed, and again cooled as earlier described. This recycling causes lower-boiling components to build up in the process, increasing the required size of the distillation tower and the cost of compression. In other cases, such as in the recovery of isobutylene, the distillation tower vapor overhead is taken directly as a secondary product and some valuable isobutylene product is lost.
Other low-temperature gas separation processes disclosed in the prior art have utilized various process configurations to separate and recover light hydrocarbons and lower-boiling components. In some cases, these light hydrocarbons have included olefins.
In U.S. Pat. No. 2,014,724, a process is disclosed for recovering olefins from petroleum refining gases and product gases from the thermal cracking of C.sub.2 to C.sub.4 paraffins. The process sequence includes compression, cooling, and phase separation to yield a gas stream of C.sub.4 and lighter components. This stream is cooled by heat exchange with other process streams and is separated by distillation, or absorption and distillation, into close-boiling fractions of paraffins and olefins having the same carbon number. Olefins are recovered from these fractions chemically and the remaining paraffins are thermally cracked and recycled to the beginning of the process.
A process for recovering olefins produced by thermal cracking of gas oil is described in U.S. Pat. No. 2,514,294 wherein the gas product from cracking is compressed, cooled, and separated into a first vapor and a first liquid stream. This first liquid stream is contacted with a heavier liquid stream from the gas oil cracker in an absorber-fractionator, from which are taken a C.sub.3 and lighter vapor stream and a C.sub.4 -C.sub.5 hydrocarbon stream containing most of the diolefins produced by the cracking process. The C.sub.3 and lighter stream is combined with the first vapor stream, caustic washed and dehydrated, and fractionated to recover ethylene and propylene products. C.sub.4 olefins and diolefins are recovered separately by distillation. The key improvement disclosed in this reference is the recovery of C.sub.4 -C.sub.5 hydrocarbons, especially diolefins, prior to the caustic treatment, dehydration, and fractionation of C.sub.3 's and lighter components.
U.S. Pat. No. 2,617,272 discloses a low-temperature separation process for the recovery of methane, ethane, ethylene, and propylene from a gas stream comprising these components with lesser amounts of hydrogen, propane, and higher boiling hydrocarbons. The process comprises compression, cooling, and partial liquefaction, followed by separation of the resulting liquid in three distillation columns in series operating at successively higher temperatures and lower pressures to recover methane, ethane plus ethylene, and propane plus propylene. The essential feature of the disclosed process is an integrated and separated propylene recirculation system for internal heat transfer and reboiling duties. Refrigeration is provided by conventional methane, ethylene, and ammonia refrigeration circuits.
A process to recover C.sub.3 to C.sub.5 hydrocarbons from natural gas is described in U.S. Pat. No. 3,542,673. In this process, natural gas is compressed to .+-.500 psi of the mixture critical pressure; this critical pressure is typically between 1600 and 3000 psia. Refrigeration is then provided to condense all or nearly all of the hydrocarbons present in the feed. This refrigerated stream, including any uncondensed components, is subjected to a series of 4 to 10 successive adiabatic flash stages wherein intermediate liquid streams are flashed at decreasing temperatures (due to autorefrigeration) and decreasing pressures. Methane and ethane are withdrawn as vapor from each flash stage. Liquid from the last flash stage is distilled to recover the product containing propane and heavier hydrocarbons. Since olefins are almost never present in natural gas, this product stream will contain essentially non-olefinic hydrocarbons.
U.S. Pat. No. 4,272,270 discloses a cryogenic process for recovery of light hydrocarbons from refinery off-gas streams comprising mostly hydrogen and containing typically 15 mol % or less of hydrocarbons. Olefins are not present in the typical refining off-gas streams treated by the disclosed process. In the process, refinery off-gas is compressed, cooled against cold product gas and other process fluids, further cooled by external refrigeration, and separated into a first vapor and a first liquid stream. The first liquid stream is sent to a stabilizer-fractionationator column; the first vapor stream is further cooled and expanded in a series of vapor-phase expander-separators, and the resulting cooled vapor and liquid streams are also sent to the stabilizer-fractionator column. This column produces a cold hydrogen vapor product which is warmed against feed and intermediate process streams, and a liquid product comprising the recovered hydrocarbons. Temperatures in this cryogenic process range from near ambient to -188.degree. F.
These prior art processes are designed to separate products from various refinery streams or natural gas. In some of these cases, olefins are present in the feed stream or are produced in the disclosed process. The types of feed streams in these cases dictate the optimum process configurations for recovery of the products of interest. The feed streams for these disclosed processes are distinctly different from those of the present invention. While the individual components are in many cases the same, the relative amounts of these components in the typical feed stream for the process of the present invention raise unique process design problems which are addressed by the present invention.